arXiv subjects

Stefano Camera

Publications and source records attributed to Stefano Camera.

111 records · Page 7Linked to original sources

arXiv2011

Testing a Phenomenologically Extended DGP Model with Upcoming Weak Lensing Surveys

A phenomenological extension of the well-known brane-world cosmology of Dvali, Gabadadze and Porrati (eDGP) has recently been proposed. In this model, a cosmological-constant-like term is explicitly present as a non-vanishing tension sigma on the brane, and an extra parameter alpha tunes the cross-over scale r_c, the scale at which higher dimensional gravity effects become non negligible. Since the Hubble parameter in this cosmology reproduces the same LCDM expansion history, we study how upcoming weak lensing surveys, such as Euclid and DES (Dark Energy Survey), can confirm or rule out this class of models. We perform Markov Chain Monte Carlo simulations to determine the parameters of the model, using Type Ia Supernovæ, H(z) data, Gamma Ray Bursts and Baryon Acoustic Oscillations. We also fit the power spectrum of the temperature anisotropies of the Cosmic Microwave Background to obtain the correct normalisation for the density perturbation power spectrum. Then, we compute the matter and the cosmic shear power spectra, both in the linear and non-linear regimes. The latter is calculated with the two different approaches of Hu and Sawicki (2007) (HS) and Khoury and Wyman (2009) (KW). With the eDGP parameters coming from the Markov Chains, KW reproduces the LCDM matter power spectrum at both linear and non-linear scales and the LCDM and eDGP shear signals are degenerate. This result does not hold with the HS prescription: Euclid can distinguish the eDGP model from LCDM because their expected power spectra roughly differ by the 3sigma uncertainty in the angular scale range 700<l<3000; on the contrary, the two models differ at most by the 1sigma uncertainty over the range 500<l<3000 in the DES experiment and they are virtually indistinguishable.

astro-ph.CO↗

arXiv2009

Constraining f(R) theories with Type Ia Supernovae and Gamma Ray Bursts

Fourth - order gravity theories have received much interest in recent years thanks to their ability to provide an accelerated cosmic expansion in a matter only universe. In these theories, the Lagrangian density of the gravitational field has the form R + f(R), and the explicit choice of the arbitrary function f(R) must meet the local tests of gravity and the constraints from the primordial abundance of the light elements. Two popular classes of f(R) models, which are expected to fulfill all the above requirements, have recently been proposed. However, neither of these models has ever been quantitatively tested against the available astrophysical data. Here, by combining Type Ia Supernovae and Gamma Ray Bursts, we investigate the ability of these models to reproduce the observed Hubble diagram over the redshift range (0, 7). We find that both models fit very well this dataset with the present day values of the matter density and deceleration parameters which agree with previous estimates. However, the strong degeneracy among the f(R) parameters prevents us from putting strong constraints on the values of these parameters; nevertheless, we can identify the regions of the parameter space that should, in principle, be carefully explored with future data and dynamical probes in order to discriminate among f(R) theories and standard dark energy models.

astro-ph.CO↗

arXiv2009

Weak lensing signal in Unified Dark Matter models

Weak gravitational lensing is a powerful tool for studying both the geometry and the dynamics of the Universe. Its power spectrum contains information on the sources emitting photons and on the large--scale structures that these hotons cross. We calculate the weak lensing cosmic convergence and shear power spectra, in linear theory and Limber's approximation, for two different classes of cosmological models: the standard \LambdaCDM and Unified Dark Matter (UDM) models. The latter models attempt to solve the problems of the dark matter in the dynamics of galaxies and galaxy clusters and of the late-time acceleration of the Universe expansion by introducing a scalar field that mimics both dark matter and dark energy. A crucial feature of the UDM models is the speed of sound c_\infty, that is the value of the sound speed at late times, on which structure formation depends. In this paper, we provide the predictions of the UDM models for the weak lensing signal,with various values of c_\infty. We consider both the Cosmic Microwave Background and background galaxies at different redshifts as sources for the weak lensing power spectra. We find that the power spectra in UDM models are more sensitive to the variations of c_\infty for sources located at low redshifts. Moreover, we find that for c_\infty>10^{-3} (in units of the speed of light), the UDM weak lensing convergence power spectrum C^{κκ}(l) for background galaxies is strongly suppressed with respect to the \LambdaCDM spectrum, particularly at small angular scales l\gtrsim100.

astro-ph.CO↗